Showing papers in "Hyperfine Interactions in 1977"

Magnetic dipolar and electric quadrupolar effects on the Mössbauer spectra of magnetite above the Verwey transition

Abstract: Mossbauer spectroscopic studies (57Fe) of powdered magnetite have been undertaken between 120 K and 880 K. Below the magnetic transition temperature (T C=839.5 K) three six-line patterns have been fitted to our experimental spectra. The broadening of the B-pattern is explained by two magnetically non-equivalent B-site irons, suggesting broadening due to electron hopping to be negligible. In the paramagnetic state the electric quadrupole splittings of iron at A-and B-sites are found to be constant, independent of temperature, having the values zero and 0.16 mm/s, respectively. The centroid shifts, on the other hand, show above 700 K large deviations from the calculated second order Doppler shift. It is proposed that the deviations arise from a variation in band overlap. The temperature variation of the magnetic fields is found to be proportional to the sublattice magnetization. The difference in the magnetic fields at the two non-equivalent B-sites is measured to be 1.1 T at 310 K.

Damage studies on the system\(^{111} \mathop {In}\limits^ \to \underline {Ni}\)

Abstract: Lattice damage after implantation of111In in Ni has been studied applying the DPAC technique to the 171–245 keV γ-ray cascade in the daughter nucleus111Cd. Implantations were carried out at 10 K and at 300 K. The low temperature implantation yields a higher regular substitutional fraction (80%) than the room temperature implantation (40%). The annealing behaviour of both implants above RT is the same. In addition, two distinct defect-associated sites were observed in isochronal annealing sequences. A microscopic model for these defects is presented, which takes into account magnetic and electric hyperfine interaction strengths, binding energies and site populations as a functions of temperature.

Large transient magnetic fields at high ion velocities in polarized iron

Abstract: Nuclear spin precessions due to the transient magnetic field in polarized Fe have been measured as a function of the initial velocity of28Si ions in the first-excited nuclear state. The transient field was found to increase linearly with the ion velocityv in the regionv/c=0.006–0.049. This is in contrast to the Lindhard and Winther model, which requires an inverse proportionality with ion velocity. Reanalysis of an earlier measurement on30Si(2 1 + ) with the linear velocity dependence yields a reduced value for theg-factor ofg=0.37±0.12. Other available velocity-dependent data for22Ne,56Fe and196Pt are consistent with a linear velocity dependence and suggest in addition a linear dependence on the nuclear charge Z of the moving ion. The increase of the transient field with recoil velocity can be explained semi-quantitatively by the capture of polarized Fe electrons into 1s and 2s vacancies in the moving ion. The velocity-dependent data and other discrepancies from the Lindhard and Winther model for16N,18O and very recently, for12C are also discussed in terms of the proposed microscopic model.

A naive model for the EFG in hep metals and numerical results for zinc

Abstract: A naive model for the charge distribution in hep metals suggests that the conduction electron charge shift, which is related to the deviation ofc/a from (8/3)1/2, is the essential source for the electric field gradient (EFG). This charge shift is derived approximately from the elastic coefficientssik of the host by application of simple electrostatics. The EFG is obtained from lattice sum calculations involving the ions and the conduction electrons in the hexagonal planes as well as between the hexagonal planes. The result for the EFG is in agreement with the “universal correlation” proposed by Raghavan et al. The anisotropy of the probe ion vibration, which averages the EFG over a finite volume, is found to contribute a dominant term to the observed temperature dependence of the EFG. Numerical results for the hep-metal Zn are in good agreement with the available experimental data. The trend of the probe deptendence of the EFG in Zn is reproduced if the effect of the probe valence on the surrounding conduction electron charge is taken into account.

Hyperfine interactions of iron implanted into aluminium

Abstract: Stable57Fe implanted into Al at energies of 20 to 70 keV and doses of 1014 to 2·1017 ions/cm2 was studied with conversion electron Mossbauer spectroscopy at room and liquid nitrogen temperatures. Spectra composed of a single line and a doublet were observed. Similarly as in the splat-quenched FeAl alloys iron monomers and iron associations, mostly dimers, are observed. The isomer shifts of both components differ considerably and do not change with iron concentration. The splitting of the doublet increases with iron concentration, the increase being reproduced by computer simulations of electric field gradients in lattices with a random distribution of charge defects. The observed probability of formation of iron associates is higher than in random systems, especially at high iron doses.

Rare-earth valence-state studies of the series RIn 3 and RSn 3 derived from quadrupole coupling constants

Abstract: Time-differential perturbed angular correlation was applied to measure the quadrupole coupling constants of111Cd (247 keV) at the In and Sn sites for rare-earth compounds of the series RIn3 and RSn3 that have the AuCu3 structure. Definitive evidence was found for Yb and Eu having lower valence states relative to the assumed value of +3 for the other rare earths. Measurements at liquid helium temperatures and pressures to 18.5 kbar indicate that the room temperature valencies of CeSn3, YbSn3, and YbIn3 have not changed.

Knight shifts and absolute magnetic moments of (h 9/2 ) n proton states in trans-bismuth nuclei

Abstract: The Knight shiftsK(FrT1) andK(RaPb) have been measured in connection with a systematic study on magnetic moments of (h9/2) n proton states. The Knight shifts were obtained directly by a comparison of precession frequencies and, indirectly, from relaxation times involving the Korringa relation. Good agreement is found among the results from the different methods; the average values areK(FrT1)=(0.89±0.09)% andK(RaPb)=(1.05±0.15)%.

Temperature dependence of the electric field gradient at Ta nuclei in hafnium pyrovanadate

Abstract: The electric field gradient EFG in hafnium pyrovanadate has been measured at Hf sites at different temperatures with the time-differential perturbed angular correlation method. The results obtained show the existence of a phase transition around 110°C.

Symmetry of some lattice-defects in nickel measured by dpac

Abstract: From DPAC measurements two defect-associated sites are known to exist after implantation of111In in nickel. Site A possesses high symmetry and does not show electric quadrupole interaction, while for site B a large axially symmetrical EFG is found. DPAC measurements on a Ni single crystal revealed that the symmetry axis of the EFG preferably points into a 〈111〉 direction. The defects are interpreted in terms of simple impurityvacancy clusters.

Effective γ-ray anisotropy in PAC-experiments with complex perturbations

Abstract: The influence of combined interactions on the effective anisotropy as observed in DPAC and in DPAD measurements is considered. Time-differential perturbation factors were calculated and compared with experimental results for111In implanted in nickel. It was found that a considerable part of the anisotropy damps out within a very short time because of lattice-damage induced electric quadrupole interaction. This may be an explanation for hidden anisotropy more generally.

Velocity and atomic number dependence of the transient magnetic field in iron

Abstract: Thev- andZ-dependence of the transient magnetic field in iron has been investigated for light ions. The present work on transient fields for20Ne and24Mg at initial velocities up tov i=8v o (v o=c/137) confirms the linearv-dependence of these fields. From the existing and present data a marked atomic shell effect has been found in theZ-dependence forZ≤26. This dependence can be described by a simple expression if the fields are assumed to be due to polarized electrons in s-shells. This shell effect can be understood qualitatively within the framework of an atomic model.

An accurate and simple method of measuring nuclear spin lattice relaxation at low temperatures, applied to60CoFe

Abstract: Pulsed eddy current heating of a thin Fe foil, cooled by direct contact with the He within the mixing chamber of a dilution refrigerator, is used to measure the nuclear spin lattice relaxation of dilute60Co in Fe. This simple method — which gives the true domain relaxation — avoids several difficulties present in other types of experiments, as is shown in a comparison of methods. The field dependence of the Korringa constant is measured between 0 and 10 kG; above 1 kG it is found to approach exponentially a limiting value of 2.27 sK.

Supertransferred hyperfine interaction in magnetic insulators (I) - Theoretical considerations

Abstract: Two additive contributions to the supertransferred hyperfine (STHF) interaction are suggested to be taken into account in its semi-quantitative analysis of the N-O-M chain: (i) that of bonding molecular orbitals, (ii) that of antibonding orbitals, by calculation of spin-polarization of s-electrons in the N-ion due to the superexchange interaction with d-electrons of the M-ion. Generalized forms of the operator and angular dependence of the STHF interaction were considered due to the use of irreducible tensor operators to the point Oh group. Analytical expressions for the value of the hyperfine fieldHSTHF at the nucleus of the N-ion induced by the magnetic M-ion (both in non-degenerateA1,A2 ground states and in degenerateE, T1,T2 states as well) are derived. The value ofHSTHF is dependent upon the type of the M-ion ground state and the geometry of the N-O-M bond. The value ofHSTHF induced by M-ions ofA1,A2,E states was shown to depend only on the bond angle θ, and that ofT1,T2 states on the combinations sin2θ cos 2ϕ, sin2θ sin 2ϕ. The importance of orbital anisotropy in the STHF interaction is emphasized. The orbital anisotropic contribution toHSTHF is drastically dependent on the ground state and its splitting in the crystalline field of low symmetry: the isotropic contribution is determined mainly by the occupation numbers of thet2g andeg subshells.

Nuclear resonance studies of 95 Nb Fe

Abstract: Dilute alloys of 35-day95Nb in polycrystalline iron have been studied by nuclear magnetic resonance on oriented nuclei (NMR/ON) using frequency modulation and fast passage techniques. Comparison of resonance frequencies with nuclear orientation results confirms the spin 9/2 for the95Nb ground state. The zero-field resonance frequency is 275.29(9) MHz, leading to ag-factor of 1.3653(25) if the hyperfine field from spin-echo measurements on93NbFe is used. Relaxation times, line shapes, and the frequencyversus field behavior are discussed. A summary of data for CoFe alloys, used here for calibration purposes, is also given.

14NII ground state hyperfine structure quantum beats via ion-beam surface interaction at grazing incidence

Abstract: Hyperfine structure (hfs) quantum beats of14NII ground terms are observed for the first time after ion beam surface interaction at grazing incidence (IBSIGI). After demonstrating the feasibility of quantum beat measurements of terms (14NII 2p3p3D) directly excited by IBSIGI the transient transfer of orientation to the nucleus after IBSIGI via hyperfine interaction is observed by measuring the circular polarization behind an additional carbon foil positioned perpendicular with respect to the beam axis. The resulting beat-structure is compared with a simulation based on a theoretical description given in this paper and is essentially ascribed to the hfs of the14NII 2s22p23P ground state. The applicability of this new method for measuring ionic ground term hfs splittings and some consequences for the interpretation of the IBSIGI interaction are discussed.

The sign of the electric field gradient at the nuclear site of Zn in Zn metal

Abstract: By use of low temperature nuclear quadrupole orientation the quadrupole coupling constant of69mZn in a Zn single crystal has been determined asvQ=eQVzz/h=−28(3) MHz

Supertransferred hyperfine interaction in magnetic insulators (II)

Abstract: Two additive contributions to the supertransferred hyperfine (STHF) interaction are suggested to be taken into account in its semi-quantitative analysis of the N-O-M chain: (i) that of bonding molecular orbitals, (ii) that of antibonding orbitals, by calculation of spin-polarization of s-electrons in the N-ion due to the superexchange interaction with d-electrons of the M-ion. Generalized forms of the operator and angular dependence of the STHF interaction were considered due to the use of irreducible tensor operators to the point Oh group. Analytical expressions for the value of the hyperfine fieldHSTHF at the nucleus of the N-ion induced by the magnetic M-ion (both in non-degenerateA1,A2 ground states and in degenerateE, T1,T2 states as well) are derived. The value ofHSTHF is dependent upon the type of the M-ion ground state and the geometry of the N-O-M bond. The value ofHSTHF induced by M-ions ofA1,A2,E states was shown to depend only on the bond angle θ, and that ofT1,T2 states on the combinations sin2θ cos 2ϕ, sin2θ sin 2ϕ. The importance of orbital anisotropy in the STHF interaction is emphasized. The orbital anisotropic contribution toHSTHF is drastically dependent on the ground state and its splitting in the crystalline field of low symmetry: the isotropic contribution is determined mainly by the occupation numbers of thet2g andeg subshells.

Isomer-shifts of i-129 impurities implanted in semiconductors

Abstract: Mossbauer spectra of the 27.8 keV transition in129I have been measured with sources of129mTe implanted in α- and β-tin and α- and β-SiC. The spectra mainly show two single line components, just as those obtained in earlier investigations with sources of129mTe implanted in diamond, silicon and germanium. The component with isomer shift corresponding to a decreased s-electron density relative to the I− ion is attributed to the substitutionally implanted impurities, that with shift corresponding to an increased s-electron density to interstitial impurities. Plots of the shifts of both component show a linear dependence on the lattice constant for diamond, silicon and germanium and α-tin. For SiC, however, the shifts are considerably smaller than expected on the basis of this linear dependence. All shifts can be quantitatively understood on the basis of a simple model that attributes the shifts of the interstitial impurities to a compression in the host lattice and shifts of the substitutional impurities to the combined effect of compression and hybridized bonding.

The problem of the Mössbauer isomer shift calibration for the119Sn resonance from rare-gas matrix isolation experiments

Abstract: Data of new rare-gas matrix isolation experiments are presented and discussed in connection with the problem of the Mossbauer isomer shift calibration for119Sn. These experiments are: (i) A Mossbauer source experiment with119mSn in solid xenon yielding the isomer shift of Sn+ with the atomic configuration 4d105s25p; (ii) Mossbauer absorption studies of isolated Sn(II) halide molecules (SnX2, X-F, Cl, Br, I) in argon matrices; and (iii) Mossbauer absorption experiments with isolated125Te atoms and molecules in argon and krypton matrices.

Mössbauer conversion electron studies of tantalum metal surface layers

Abstract: Conversion electrons following the Mossbauer absorption of the 6.2 keV γ-rays of181Ta were observed in backscattering geometry. Mossbauer spectra for tantalum single crystal and foil surfaces (mean-depth 330 A) are compared with normal transmission spectra for tantalum foils (bulk). While no broadening of lines is observed for carefully polished single crystal furfaces, foil surfaces show considerably broader lines than bulk spectra. The linewidth and isomer shift indicate an increase of the concentration of absorbed residual gases at the foil surfaces. The observed dispersion term arising from the interference between photo- and conversion electrons for this E1-transition, depends only on the absorber thickness.

The depolarization and diffusion of μ + in chromium

Abstract: Precession of polarized positive muons is used to probe internal magnetic fields in a polycrystalline sample of high purity chromium. A depolarization, or damping in time, of the precession oscillation is observed in the antiferromagnetic states between 77 K and 312 K. The results are used to obtain the temperature dependence of the muon diffusion rates, with the assumption that the local fields are of dipolar origin.

Electric field gradient in pure hexagonal transition metals

Abstract: Using a general calculation of the nuclear quadrupole interaction in non-cubic metals which was presented in a previous paper, this article gives an interpretation of experimental data dealing with signs and temperature dependence of the electric field gradient in 3d(Sc, Ti), 4d(Y, Zr, Tc, Ru) and 5d (Hf, Re, Os) transition hexagonal close-packed metals

Temperature dependence study of the quadrupole interaction of 111 Cd in antimony

Abstract: A power law dependence has been observed for the temperature dependence of the quadrupole interaction of the 247 keV state of111Cd in antimony, which is similar to that observed for121Sb and123Sb in antimony.

Mossbauer-effect studies of recovery of iron after heavy-ion implantation at 7-k

Abstract: Mossbauer effect measurements were performed with sources of133Xe implanted at 7 K in polycrystalline iron foils by means of an isotope separator. Information about the direct environment of an implanted radioactive probe atom is obtained through the hyperfine interaction of the daughter nucleus (133Cs) with its surroundings. The Mossbauer spectra were analyzed on the basis of an earlier model, assuming three visible spectral components with high, intermediate and low hyperfine fields and recoilless fractions, respectively, and a fourth, invisible component with a recoilless fraction very close to zero. Annealing experiments showed no evidence for post-implantation recovery stage I in the iron foils, while recovery stage II only gave rise to small changes in the site occupations. These results can be explained in terms of the high energy density locally deposited in the lattice by each implanted atom, leading to stage I recovery during the very short time that is necessary for the thermal dissipation of this energy. Quantitatively, these results are confirmed by calculations of Sigmund on heavy-ion induced elastic-collision spikes in solids. After annealing at room temperature the site occupations agree with those obtained directly with room temperature implanted sources. In previous experiments we have shown that different results are obtained for annealing and for implantation at 200°C. From these results we estinate the migration energy of vacancies in iron at 1.27±0.09 eV.

Interaction between recoil-implanted111In impurities and radiation damage in silver

Abstract: We report a study of the impurity-radiation damage interaction for In in silver. Time-differential perturbed angular correlation experiments were carried out on111Cd, the radioactive daughter of111In produced and recoiled in Ag following the109Ag(α, 2n)111In nuclear reaction. All irradiations were performed at 80 K. The dose-rate and annealing temperature dependences of the defect-induced quadrupole interaction on111Cd were investigated. Clear differences are observed between the effects of correlated damage (the impurity interacting with the damage created by its own recoil) and uncorrelated damage (where the impurity is submitted to the influence of other damage cascades). In the former case (low dose-rate experiments), about 6% of the111In impurities are subject to a well-defined quadrupole interaction which is ascribed to a near-neighbor vacancy. This proportion is temperature-independent up to 500 K, but the amplitude of the quadrupole frequency is reduced by a factor of three between 300 and 400 K. Assuming vacancy release by the impurity, this would lead to a binding energy of 0.34(15) eV. The remainder of the111In nuclei are subject to a distribution of quadrupole interactions presumably due to a statistical repartition of lattice defects at some distance from the impurities. The width of the distribution changes upon annealing: a clear transition takes place at 200 K, which is the divacancy mobility threshold in Ag deduced from resistivity experiments. When cascade overlap occurs (high dose-rate experiments), no impurity-defect trapping is evidenced but a distribution of quadrupole interactions due to surrounding damage is still found. From these experiments, an order of magnitude of the damage cascade “size” is deduced. Our results are discussed in terms of available information on the impurity-damage interaction in various hosts; a compilation of all results on the impurity-vacancy interaction known to us is also presented in the paper.

Hyperfine interaction of161Dy imprities implated in iron and nickel

Abstract: In this paper we present a full report on our Mossbauer measurements on implanted sources of161TbFe and161TbNi using the 26.7 keV Mossbauer transition in161Dy. From the measured spectra values were derived for the magnetic and quadrupole interaction strengths of rare earth ions in at least two non-equivalent lattice sites and information is obtained about the relaxation behaviour of these ions. Conclusions are drawn about the lattice location of the implanted ions. Information could also be obtained about the crystalline electric field acting upon the substitutionally implanted rare-earth ions.

Nuclear orientation of105Rh and95Tc in iron

Abstract: The temperature-dependent anisotropy of γ-rays following the decay of oriented95Tc and105Rh nuclei was studied with a Ge(Li) detector. Mixing coefficients of some γ-and preceding β-transitions, the spins of two intermediate levels, and the magnetic hyperfine splitting of the95Tc and105Rh ground states in an Fe host were measured. From the known hyperfine fields the following magnetic moments were deduced:

Gyromagnetic ratios of first 2 + states in 126, 128, 130, 132 Xe

Abstract: Gyromagnetic ratios of first 2+ states in126, 128, 130, 132Xe were determined by implantation perturbed angular correlations (IMPAC). The effective hyperfine magnetic field together with the transient magnetic field at xenon nuclei in iron was utilized to obtain the precession of the angular correlation. The precession due to the transient field was taken from systematics to be φ/g=−36±6 mrad. The effective hyperfine magnetic field was determined in an experiment on126Xe in iron to be 900±200 kG. The results for theg-factors areg(128)=0.41±0.07,g(130)=0.38±0.07 andg(132)=0.37±0.05. Theg-factor of the 2+ state in126Xe was determined in a separate experiment using a radioactive source of126I to beg(126)=0.37±0.07, and was used as calibration for the IMPAC-data.

A new approach to stroboscopic resonance measurements of the nuclear quadrupole interaction

Abstract: A new stroboscopic resonance method for the measurement of the quadrupole interaction of long-lived nuclear isomers excited by pulsed beams with a fixed repetition frequency is demonstrated. The method utilizes the phenomenon of beats which occurs when a magnetic perturbation is introduced in the presence of a pure quadrupole interaction. The nuclear precession is thus controlled by the beat frequencies which can be varied by the external magnetic field which is oriented parallel to the symmetry axis of the crystal. Stroboscopic resonance can be achieved by varying thebeat frequency to be an integral multiple of the beam pulsing frequency. The method is demonstrated using the 398 keV level of69Ge excited and recoil implanted into a single crystal of tin by means of a heavyion reaction.

In-beam nuclear magnetic resonance in isomeric states

Abstract: A comprehensive description is given of the γ-NMR-PAD method combined with pulsed beam techniques. Detailed studies of the resonance behaviour were performed as a function of various timing conditions, of the phase between the r.f. field and thecreation time of the nuclear states, and of the amplitude of the linearly polarized r.f. field. The following features were realized by experiments on the 20 ms state in71Ge and on the 119 μs state in78 Br, excited and oriented by nuclear reactions in liquid metallic targets: (i) The resonance behaviour was observed time-differentially, yielding accurate values of the r.f. amplitude acting at the nuclear site, and of the relaxation time. (ii) The observation of the perturbed γ-ray distribution in time windows can yield strongly structured resonances with considerably larger effects compared to time-integral resonances. The time window technique is well suited to determine the resonance point precisely, and further — in case of saturation — the relaxation time can be extracted. (iii) Absorptionand dispersion-like resonance structures were observed with fixed r.f. phases for selected time windows and phase positions, yielding the sign of the magnetic interaction and the sign of the γ-ray anisotropy. (iv) At r.f. amplitudes comparable to the static magnetic fields resonance shifts and multiple quantum effects caused by the “nonresonant” component of the r.f field were studied in γ-NMR-PAD. A comparison is given with the stroboscopic method, and some possible applications of the technique developed are discussed to observe hyperfine interactions of muons in non-metals.